BHP gets ’em talking with Olympic Dam leaching

BHP Billiton
’s decision to go back to the drawing board on the Olympic Dam copper-uranium expansion and to consider leaching rather than traditional processing has piqued the mining industry’s curiosity.

That’s because, if approved, it would represent the first commercial-scale leaching of copper from chalcopyrite ore. In hydrometallurgical circles, that has long been considered the “holy grail", as more than 70 per cent of global copper reserves are chalcopyrite.

To date leaching has been done very successfully on oxide ore and more recently chalcocite ore at copper mines around the world. Then the ore is processed through a solvent extraction and electrowinning (SX/EW) process which avoids the need to build a smelter and use the more traditional pyrometallurgical process.

In his discussions about the future of Olympic Dam, BHP chief
Marius Kloppers
has given a few clues as to the likely leaching process.

He says the leaching cycle is likely to take 300 days or more, but the recoveries to date, based on lab work, columns and even test cribs, have been good so far.

In leaching, recoveries are critical. Alta Metallurgical Services’s Alan Taylor says recoveries through that method would tend to be in the 70 to 90 per cent range. That is lower than the high 90 per cent range typical through smelting, but infrastructure construction costs are lower.

Taylor and other metallurgy experts said in the case of Olympic Dam, the ore would probably be crushed and stacked on heap leach pads and recovered through the use of acid and specialised bacteria. The uranium should be easily recoverable as well through this process. While a full leach cycle could take 300 days or more, production from some pads would begin much earlier.

But to get to the stage where BHP would be comfortable on signing off on a full investment – especially because a large open pit still needs to be dug at the site – the miner would likely want to construct a pilot plant. The cost would depend on the scale, but it could easily reach the $100 million mark in this case.

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And in terms of time frame, Kloppers suggested it would be measured in years rather than months. Factoring in testing and environmental approvals, industry experts say, it would probably be a minimum of two to three years before BHP would be ready to approve the investment.

Leaching tests at Olympic Dam date to earlier in the 2000s, when WMC examined the opportunity before it was bought by BHP. Clearly BHP has not perfected the process or it would have been ready with a scaled-down plan by last week.

One question is the amount of annual tonnage BHP will look to process through leaching. It had been expected to process 60 million tonnes of ore from the open pit expansion. One of the largest leaching operations in the world, Talvivaara’s nickel project in Finland, expects to do 18 million tonnes of ore this year but has faced teething problems. Based on lower recoveries and the available scale, BHP’s production growth plans for Olympic Dam may need to be scaled back a bit.

Leaching of chalcopyrites is being researched by most of the world’s major copper miners.
Rio Tinto
, for example, plans to use leaching at its La Granja project in Peru, which has a chalcocite cap on top of chalcopyrites. The initial stage will target the chalcocite, which is easier to extract through that method.

BHP itself is examining chalcopyrite leaching for future expansions at its Cerro Colorado mine in Chile and has been conducting some high temperature leaching test work in South Africa.

In Australia, the BioHeap division of nickel miner
Western Areas
has been working on commercialising its technology, which perhaps uniquely allows the use of saline water rather than fresh water.